Due to the increasingly high prices of hydrocarbon raw materials coupled with their gradual exhaustion, a worldwide intensive search is underway for alternative sources of energy. One of the most efficient and ecological ways to generate power is to use the energy of the chemical reaction H2+½O2=H2O. In order to convert the energy of this chemical reaction directly into electrical energy a special arrangement called a fuel cell is required.
Depending upon the electrolyte used, several different types of fuel cells exist. The different fuel cells include fuel cells with solid electrolyte (SOFC), fuel cells with polymer electrolyte (PEFC), fuels cells with acid electrolyte (for example, phosphoric acid fuel cells (PAFC)), molten carbonate fuel cells (MCFC), and alkaline electrolyte fuel cells (AFC).
AFC's were one of the first types of fuel cells to be applied to a practical use. For example, the spacecraft industry uses AFC's. The principle of operation of an AFC is well known, and illustrated in FIG. 1.
Electrodes for AFC's are sophisticated technical devices and are made of several components. For example, the cathode for an AFC typically comprises at least a diffusion layer of polytetrafluoroethylene (PTFE) and high surface-area carbons and an active catalytic layer. Platinum group metals (e.g., Pt or Pd) are widely used as catalysts for the reduction of oxygen in an AFC. Primary disadvantages to these types of catalysts include their steep price and sensitivity to chemical pollutants.
In response to these disadvantages, different oxides have been proposed as catalysts for oxygen reduction electrodes. They include, for example, MnO2 and oxides with spinel-type structures such as CoxFe3−xOn or CoxNi3−xOn. Another group of catalysts include perovskite-type materials. In the crystal structure of the perovskite ABO3, the A− cation and O2− anions form a cubic close packing structure. Smaller B− cations, like transition-metal cations (e.g., Fe, Co, Ni) occupy octahedral holes in the close packing structure.
Depending upon the concentration of oxygen vacancies in ABO3−y, the ordering of vacancies may take place leading to the formation of the ordered perovskites. For example, a majority of the ABO2.5 oxides crystallize in brownmillerite-type structures where layers of corner-shared octahedra are separated by oxygen-deficient layers containing chains of tetrahedra.
Transition-metal compounds with perovskite structures have high electronic conductivity and can be used as catalysts for the oxygen reduction electrodes in AFC's. Given these properties, complex oxides with perovskite structures have been studied as potential catalysts. For example, nickel- and cobalt-based perovskites of the following formula have been studied:Ln1−xAxCo1−yNiyO3−δ  (1)wherein 0≦x≦0.6; 0.01≦y≦0.1;0≦δ≦x/2, and wherein Ln represents an element of the group consisting of La, Pr, Nd, Sm, Gd, and Y, and wherein A represents an element of the group consisting of Ca, Ba, and Sr. The perovskites of Formula 1 demonstrate high catalytic activity.
Also, perovskites of the following formula:ABO3−δ  (2)wherein −0.2≦δ≦−0.05 and +0.05≦δ≦+0.7, and wherein A represents an element of the group consisting of Na, K, Rb, Ca, Ba, La, Pr, Sr, Ce, Nb, Pb, Nd, Sm, and Gd; and wherein B represents at least one metal from the group consisting of Cu, Mg, Ti, V, Cr, Mn, Fe, Co, Nb, Mo, W, and Zr, were shown to have high catalytic activity in the process of oxygen reduction.
Compounds with a brownmillerite structure:ABO2.5−y  (3)wherein −0.2 ≦y≦−0.05 and +0.05 ≦y≦+0.3, were also found to be good catalysts.
Perovskites of the following formula:(AxB1−x)(C)O3+(−)y  (4)wherein A represents an element of the group consisting of Ca, Sr, and Ba; and wherein B is any one element of atomic numbers 57-71; and wherein C represents any one element of atomic numbers 40-47 and 72-79, were also studied as cathode catalysts for AFC's.
Also studied were the following perovskites:AxByO3  (5)wherein 0.1≦x≦0.9; 0.1≦y≦0.9, and wherein A represents an element consisting of the group of Ba, Sr, Ca, Y and Sc and/or an element consisting of the group of La, Ce, Sm, Pr and Nd, and wherein B represents one or several transition metals of the group consisting of Co, Mn, Fe, Ni, Cu, Cr, Pd, Pt, Ru, Rh, and Ir.
Finally, high catalytic activity also was reported for perovskitesA1−yByQO3  (6)wherein (A=La, B=Sr, Q=Co, y=0.3); (A=Ce or Sc; B=Sr or Mg; Q=Ni, Co, or Mn; 0.2≦y≦0.4); or (A=La, Ce, Nd, Pr,or Sc; B=Sr, Ca, Ba, or Mg; Q=Ni, Co, Fe, or Mn; 0.0001≦y≦1).
One of the most important traits for the materials used as catalysts for oxygen reduction in AFC's is high chemical stability. Compounds with perovskite-like structure have high catalytic activity but poor stability in alkaline solutions. For example, the activity of LaNiO3 was found to decrease rapidly in alkaline solutions.
Therefore a need exists for a catalyst for oxygen reduction in AFC's that demonstrates high catalytic activity and high chemical stability.